In 3GPP (3rd Generation Partnership Project) which is a project aiming to standardize a mobile communication system, the introduction of Device to Device (D2D) communication is discussed as a new function after Release 12 (see a non-patent document 1).
In the D2D communication, a plurality of user terminals adjacent to one another perform direct radio communication in a frequency band assigned to a mobile communication system. In addition, the D2D communication is also called Proximity Service communication.